End terminations for variable resistance elements



Feb. 25, 1969' A. ,1. WRIGHT 3,430,178

END TERMINATIONS FOR VARIABLE RESISTANCE ELEMENTS Filed Nov. 5, 1966 Sheet of 2 INVENTOR.

ALAN J. WRIGHT Y ATTORNEY Feb. 25, 1969- A. J. WRIGHT 3,430,178

END TERMINATIONS FOR VARIABLE RESISTANCE ELEMENTS Filed Nov. 5, 1966 Sheet 2 012 ATTORNEY United States Patent 3,430,178 END TERMINATIONS FOR VARIABLE RESISTANCE ELEMENTS Alan J. Wright, Orange, Califi, assignor to Beckman Instruments, Inc., a corporation of California Filed Nov. 3, 1966, Ser. No. 591,732 US. Cl. 338-162 Int. C. Htllc 5/02, 1/12, 1/14 3 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to electrical resistance elements and to a method of making such elements and is more particularly directed to a resistance element of the type used in a variable resistance device having a movable electrical contact or wiper adapted to traverse the resistance element.

The invention is primarily applicable to resistance elements and devices using a resistance material which is applied as a layer or film on a nonconductive support or base. One such layer is formed of a mixture of glass and metal particles and commonly called cermet resistance material. A number of typical cermet resistance films and methods of making the same are described in US. Patent Nos. 2,950,995 and 2,950,996 issued in the name of Thomas M. Place, Sr. et a1. and assigned to the same assignee as the present invention. Another resistance material suitable for such resistance devices is the well-known conductive plastic material, which is formed of a plastic binder material having dispersed therein conductive metal particles of carbon or other electrically conductive material.

The resistance layer and method of the present invention is preferably used in rotary potentiometers and variable resistance devices having full 360 or continuous rotation of the wiper or contact member. However, the invention is applicable to resistance layers deposited in any shape or form. It is normal practice, in 360 rotational devices, to provide an annular or arc-shapedJayer of resistance material having a gap or open space between the ends of the resistance layer. At the respective ends of the layer on opposite sides of the gap are provided end terminals or electrical connections through which are electrical current is applied to the resistance layer. The dead space or gap is usually filled with a suitable film of nonconductive material which accommodates passage of the wiper or movable electrical contact thereover as it is moved from one end of the resistance material to the other. It has been found that, as the wiper passes from the resistance layer proper to the nonconductive material in the dead space or gap, during continuous rotational operation, an are sometimes occurs due to the abrupt change in potential as the wiper traverses the electrically active portions of the resistance element to the dead space. This arcing tends to erode resistance film and to shorten the service life of the element.

Accordingly, it is an object of the present invention to provide a means of terminating the resistance layer in a manner which will reduce the eroding effects of arcing and increase the service life of the resistance layer.

3,430,178 Patented Feb. 25, 1969 Further objects and advantages of the invention will become apparent as the following description proceeds and the features of novelty which characterize the invention will be pointed out with particularly in the claims annexed to and forming a part of the specification.

In carrying out the objects of the present invention, there is provided a resistance element in the form of a nonconductive base member having a flat surface area upon which is deposited a resistance layer having a gap therein. The gap in the resistance layer may be filled with a nonconductive film of material forming a bridge section across the gap. In order to prevent deterioration of the resistance layer in this region as the current carrying wiper traverses from the resistance layer to the bridge section and vice versa, end terminations :for conducting current to the rseistance layer are provided and are so constructed and arranged as to cause arcing to occur only between the wipers and the end terminations and not upon the resistance layer. These end terminations are composed of a metal or metal composition which will more adequately withstand the detrimental eifects of arcing.

The novel features which are believed to be characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, together with further objects and advantages thereof can best be understood by a reference to the following descriptions taken in connection with the accompanying drawing in which:

FIGURE 1 is an isometric view of an exemplary resistance element embodying the present invention;

FIGURE 2 is an enlarged top view of the resistance element in FIGURE 1 illustrating in greater detail the low resistance connection pads, end terminations, contact wiper, and the manner in which contact is made as the contact wiper rotates;

FIGURE 3 is an elevation view, in section, taken sub stantially along line 33 of FIGURE 2, illustrating the lead wires or pins which abut onto the end terminations;

FIGURE 4 is an isometric view of a resistance element illustrating another embodiment of the invention;

FIGURE 5 is an enlarged partial top view of the resistance element of FIGURE 4 illustrating in greater detail the terminations, termination pads, and the contact wiper; and

FIGURE 6 is a cross-sectional view taken substantially along line 66 of FIGURE 5, illustrating the lead wires or pins which abut onto the end terminations.

Referring now to FIGURES l, 2, and 3 there is shown a resistance element of the type adapted for use in a variable resistance device. The resistance element comprises a substrate base member 1 of dielectric material which may be in the form of :a circular or rectangular disc or water. The base member 1 is usually formed of an unglazed, homogeneous, non-porous, ceramic material, such as steatite or alumina, or other nonconductive material well known in the art. While the illustrated device is in the form of a circular disc of the type used in rotary turn potentiometers or rotary variable resistors, it will be understood that the present invention is not limited to resistance devices of this particular shape and that the invention is equally applicable to resistance devices formed of rectangular shape or any other suitable configuration.

In conventional practice, the substrate or base member 1 is molded, fired and then ground or lapped to provide a smooth relatively flat surface for supporting a resistance [film or layer 2 thereon. The resistance layer 2 forms a conductive path for an electrical current which is applied to the resistance layer through terminal members 3. In the embodiment of the invention illustrated in FIGS. 1, 2, and 3 the terminal members or conductors 3 are in the form of metal ribbons or leads imbedded in the base member 1. The noble metals, because of their resistance to oxidation, have been found best suited for use as terminal conductors and various alloys of gold, silver, platinum,paladium or rhodium are available with a coefiicient of expansion which matches, to some extent, that of the ceramic base member 1.

The resistance film or layer 2 may be formed of a suitable conductive or resistance material such as conductive plastic material having a carbonaceous material dispersed throughout a fused plastic. It may also comprise a cermet resistance material formed of a nonconductive glass binder material having minute particles of noble metal or other electrically conductive metal alloy dispersed throughout the fused glassbinder. In cermet materials, the mixture of glass and metal particles, after fusion at a temperature below that of the metal constituent, forms a continuous element having a hard, smooth, glassy surface. Particular examples of cermet materials and their method of manufacture are fully explained in the aforementioned Place at al. Patent Nos. 2,950,995 and 2,950,996 assigned to the same assignee as the present invention.

The resistance material is applied to the base member 1 in a configuration desired for the particular application of the resistance device. For example, in the disclosed disc-type element, the film or layer is annular or arc-shaped and may be applied to the base member 1 by any suitable method well-known in the art, such as brushing, spraying, stenciling or silk screening. Those areas on the surface of the substrate or nonconductive base member upon which the material is not desired are masked in a manner well-known in the art in order to prevent the disposition of a coating thereon. A space or terminal gap in the base member, between the terminal members 3, is suitably masked so that, when the resistance material is applied, the gap is left vacant.

Within the space may be deposited a bridge film or layer 7 of nonconductive material, such as glass or plastic having no conductive particles therein. This nonconductive layer is of substantially the same thickness as that of the resistance material in order to provide a smooth transition of the wiper from one end of the resistance layer to the other. The glass or plastic material in the bridge area should be similar to that used in the formation of the resistance material so that the materials in the bridge area fuse at the same or similar temperatures as the resistance material. Thus, the nonconductive film or layer 7 ispreferably formed of glass when the resistance material is cermet and may be of plastic when the resistance material is conductive plastic.

The end terminations 3 are electrically connected to the resistance layer 2 by means of low resistance connection pads 4 which are typically composed of a silver or other conductive metallic paste which is hardened by firing. As illustrated in FIGURE 3, conductive lead wires or terminal pins 6 may be provided for connecting the resistance layer into an external circuit. In the embodiment illustrated in FIGS. l3, terminal pins 6 abut onto and are in electrical contact with the lower surface of the connection pads 4 and extend downwardly through the base member 1.

The contact wiper 5, which may typically be formed of silver or silver alloy on a beryllium-copper spring arm, is here shown with a number of resilient fingers a disposed on a radial line from the center of the circular base member 1 while end terminations 3 are disposed at an angle differing from that of the wiper 5. In such a configuration, the contact wiper fingers 5a touch the end termination 3 before contacting the resistance layer 2 and leaves the end termination 3 only after breaking contact with resistance layer 2. The angle between the contact wiper fingers 5a and the end termination 3 is such that the contact wiper fingers 5a touch both the end termination 3 and the resistance layer 2 before the wiper fingers disengage from the end termination 3. Since the end termination 3 and the resistance layer 2 are electrically connected by connection pads 4, arcing occurs only between the contact wiper 5 and the end terminations 3 as the wiper fingers 5a touch and disengage from the end terminations 3 rather than occurring between the resistance layer 2 and the wiper fingers 5a.

Referring now to FIGURES 4, 5, and 6, there is shown another exemplary embodiment of the invention including a variable resistance element comprising a resistance layer 11 of cermet or other suitable material deposited on the upper surface of the base member 12. The composition of all components described in this embodiment may be determined, unless otherwise specified by reference to the description of the embodiment illustrated in FIG- URES 1, 2, and 3.

The resistance layer 11 is provided with at least one end termination 13 which abuts against or overlaps the end of the resistance layer 11 on one side thereof. Electrically connected with the end terminations 13 are termination pads 14 which are substantially wider than the end terminations 13 and extend radially beyond the nonconductive bridge area 15. FIGURE 5 depicts these termination pads 14 as being disposed along the outer diameter of the path circumscribed by the resistance layer 11, but it will be understood that the termination pads 14 may also be located along the inner edge of the path circumscribed by the resistance layer 11.

The end terminations 13 are typically composed of material such as a metal-glass paste of silver or gold-glass mixtures or other compounds well-known in the art. The termination pads 14 may be composed of materials similar to that provided for the end terminations 13- or may consist of a more durable material such as rhodium or platinum.

As illustrated in FIGURE 6, conductive lead wires or pins 20 are provided which electrically contact the lower surface of the termination pads 14 and extend downwardly through the base member 12 to provide a means for connecting the device into an electrical circuit. Other terminal connectors may be employed such as wire terminals suitably attached to the termination pads 14 by welding, soldering, or some suitable mechanical connection.

The contact wiper 16 (only partially shown), includes a plurality of contact fingers 17 which ride on the resistance layer 11 and on the end terminations 13. The wiper also includes at least one additional finger 17a, adapted to traverse outside the resistance track yet make contact with the termination pads 14. Whether the termination pads 14 are disposed along the inner or the outer edge of the path circumscribed by the resistance layer 11, they will be located and formed in such manner that wiper fingers 17 leave the end terminations 13 before the wiper finger 17a disengages the from the termination pads 14 and so that wiper fingers 17 contact the end termination 13 after wiper finger 17a contacts the termination pad 14. In the preferred arrangement, the pads 14 are much wider than the end termination 13 and extend in the direction of the bridge section 15. Therefore, since the termination pads 14 and the end terminations 13 are electrically connected, arcing occurs only between the contact wiper finger 17a and the termination pads 14 as the wiper finger 17a touches or disengages from the termination pads 14 rather than occurring between the end terminations 13 and the Wiper fingers 17. This prevents deleterious arcing in the region of the resistance element thereby preventing deterioration of the resistant element as a result of such arcing.

While in accordance with patent statutes there has been described what at present is considered to be the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention and it is, therefore, the aim of the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is: 1. A variable resistance device including an electrical resistance element traversed by a movable electrical wiper contact for varying the electrical output of said device comprising:

a nonconductive base member having a substantially smooth surface area thereon;

a layer of resistance material deposited on said fiat surface of said base member, said layer of resistance material having a gap therein;

a movable electrically conductive wiper having a plurality of aligned contact points adapted to traverse the surface of said layer, said electrical wiper contact points being disposed at a first angle to the path circumscribed by said layer of resistance material; and

at least one conductive end termination at one end of said resistance layer, said end termination being spaced from said end of said layer of resistance material and electrically connected to said layer of resistance material by means of one or more resistance connection pads deposited on said base member outside the path of excursion of said wiper contact points, said end termination being disposed at a second angle differing from said angle of said wiper contact points so that said wiper contact points touch said end termination before contacting said layer of resistance material and disengage from said end termination only after breaking contact with said resistance material.

2. A variable resistance device including an electrical resistance element traversed by a movable electrical wiper contact for varying the electrical output of said device comprising:

a nonconductive base member having a substantially smooth surface area thereon;

a layer of resistance material deposited in an annular track on said flat surface of said base member, said layer of resistance material having a gap between adjacent ends thereof;

at least one conductive end termination :for said resistance material adjacent to said gap therein and electrically connected to said layer of resistance material by overlapping or merging part of said end termination with said layer of resistance material;

a movable electrically conductive wiper having a plurality of aligned contact points adapted to traverse the surface of said layer, at least one of said aligned contact points being disposed externally of said annular path of said layer resistance material, said electrical wiper contact points being disposed at a first angle to the path circumscribed by said layer of resistance material; and

at least one termination pad abutting and electrically connected to said end termination, a portion of said termination pad being disposed externally of said annular track of resistance material and in the path of said one contact point of said wiper, said external portion of said termination pad extending :further into said gap between said ends of said layer of resistance material than does said end termination extend, said termination pad being disposed at a second angle differing from said angle of said wiper contact points so that said one contact point of said wiper touches said external portion of said termination pad before contacting said layer of resistance material and disengages' from said external portion of said termination pad only after breaking contact with said layer of resistance material.

3. An apparatus as defined in claim 3 in which said external portion of said termination pad is positioned along the outer diameter of said nonconductive base member.

References Cited UNITED STATES PATENTS 432,131 7/1890 Flemming 338-171 X ROBERT K. SCHAEFER, Primary Examiner.

H. J. HOHAUSER, Assistant Examiner.

US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,430,178 February 25, 1969 Alan J. Wright It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6, line 27, the claim reference numeral "3 should read 2 Signed and sealed this 14th day of April 1970.

(SEAL) Attest:

WILLIAM E. SCHUYLER, IR.

Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer 

